a cube has all sides the same length, check the picture below.
since the cube is just 6 squares stacked up to each other at the edges, we can simply get the area of each square and sum them up, and that's the surface area of the cube.
(3.5 · 3.5) + (3.5 · 3.5) + (3.5 · 3.5) + (3.5 · 3.5) + (3.5 · 3.5) + (3.5 · 3.5) .